The present invention relates to a device for measuring the rotary frequency of a rotating vehicle wheel in which along a circular line in the circumferential direction of the vehicle wheel poles are uniformly distributed. A sensor responsive to the poles is positioned at a distance to the axis of rotation of the vehicle wheel stationarily in the vicinity of the circular line so that the poles passing the sensor produce a sequence of signals. A computing or processing device is provided which determines the time period between passing of sequentially arranged poles on the circular line. The time periods measured and the number of poles provided at the vehicle wheel provide a means for determining the rotary frequency which is displayed on a display unit or used for slip control in an anti-lock brake system or traction system.
Such devices are well known and are applied in conjunction with wheel slip control systems especially for avoiding locking of wheels during braking.
A vehicle wheel in the context of this application includes the combination of all components which, aside from the load-depending deformations, are designed for rotation relative to the wheel suspension but are connected to one another so as not to rotate relative to one another. A wheel thus includes especially the tire, the wheel rim with rim flange and rim dish, the valve, the hub, possibly also sealing and/or securing rings mounted there at, brake discs, anti-lock system magnet wheels and optionally the drive shafts.
The goal of the invention is an increase of the safety level of motor vehicles on wheels with tires, especially pneumatic tires, which, at least in the longitudinal direction, transmit forces onto the road etc. only by frictional connection. Even though under most operating conditions the maximum possible frictional force transmission is never completely used, it is necessary to obtain great accelerations, especially with negative sign, i.e., great braking forces whenever it is necessary to react to unpredictable events, for example, when encountering out-of-control vehicles or a child running onto the street.
It is known that the level of greatest possible acceleration depends substantially on the frictional coefficient between the tire and the road surface. It is furthermore known that this frictional coefficient depends on the material pair road surface/tire, in general an asphalt/rubber mixture, the air pressure within the tire, the footprint length, the tread design as well as the weather conditions. Furthermore, it is known that the frictional coefficient is a function of slip. Slip is defined as the difference of tire circumferencial velocity minus velocity of the steering knuckle, divided by the velocity of the steering knuckle.
FIG. 1 shows for a conventional material pair under typical operating conditions the course of the frictional coefficient .mu..sub.longitudinal as a function of slip, in the following referred to as slip curve. The maximum longitudinal frictional coefficient is reached for a slip value of approximately 10%. When the slip is further increased, for example, during braking by increasing the braking moment, the frictional coefficient and thus the effective longitudinal force would no longer increase but instead would decrease. This not only would result in an immediate decrease of the braking forces but also, when maintaining the braking moment which is too great, to a fast decrease of the vehicle wheel rotary frequency and thus would cause the vehicle wheel circumferential velocity to fall to 0. This happens the quicker the smaller the moment of inertia of the wheel relative to the vehicle mass. The operational state in which the wheel no longer rotates despite the still present steering knuckle velocity is called locking. The slip then is -100%.
FIG. 2 shows in a solid line the slip curve for the same tire on cold ice (for warm ice it is even more unfavorable), and, as a comparison, in a thin dashed line the slip curve of FIG. 1 is also shown. The value .mu..sub.max is not only much lower but also occurs at a smaller slip value.
Even for a minimally excessive braking moment the rotary deceleration of the wheel surpasses the vehicle deceleration which will worsen the braking slip that initially is only minimally too large. From a starting value of, for example, -40%, it quickly decreases to -100%. Due to this effect of surpassing the slip correlated with the maximum frictional coefficient, the range outside of this slip is conventionally referred to as instable slip area. The slip area between 0 and this value is referred to as stable. The slip correlated with the maximum frictional coefficient is called critical slip.
The same holds true also for drive slip that is excessive. Slipping drive wheels have a negative effect on the vehicle safety, even though not to such a great extent as locking wheels during braking. Furthermore, in the conventional non-locking differentials, the drive force does not break down, as during braking, for each wheel but for each axle because the greater portion of the drive output is transmitted to the slipping wheel. For non-locking interaxle differentials the drive force would even break down almost completely.
In addition to the decrease of the transmittable longitudinal force, for braking under locking conditions as well as for slipping drive wheels, the vehicle safety in such operational states is impeded by the loss, in the case of locking brakes the complete loss, of the ability to transmit lateral forces. The straight running stability is thus only supported by the translatory inertia of masses and the moment of inertia about the vertical vehicle axis; steering maneuvers are impossible.
Because of the great importance of adjusting the correct slip for the requirement of greatest possible positive or negative acceleration and because of the fact that a person as a controller is usually only capable of simultaneously maintaining a maximum of two wheels within the optimal slip range, as is the case with a motorcycle, whereby for all other motor vehicles including airplanes, in general, only one actuating device for the entire number of wheel brakes is present, the development of slip control systems, i.e., of systems where a technical device takes over the control function performed by the human being, began in the 1940's initially only for the braking systems of airplane landing gear. When using such systems, a person by adjusting the lever pressure, lever travel or pedal pressure or pedal travel etc. only transmits his desire for controlling the nominal value, for example, the brake acceleration.
The control system, on the other hand, has been assigned the task to adjust for each wheel individually the favorable slip. Most of the slip control systems will only function when at one wheel almost the critical slip has been reached. By preventing a further increase of the brake, respectively, drive moment, locking or slipping is prevented. Once the critical slip has been surpassed, for example, when the wheel suddenly encounters a worse frictional pairing, as, for example, blue basalt etc. the control system reduces the brake, respectively, drive moment to such an extent and for such a period of time until the slip has been adjusted to just below the critical slip.
Slip control devices have been used for approximately nine years in the mass production of passenger cars, trucks, and trailers with increasing market share. Insofar as they control only the brake slip, the acronym ABS (derived from Anti Blocking System) has been used. Slip control systems prove their effectiveness especially impressively under such driving conditions where one wheel track runs on a surface with bad maximum coefficient of friction while the other wheel track runs on a surface with high maximum coefficient of friction.
Slip control systems of the prior art detect very precisely the actual rpm of each wheel. For this purpose, each wheel is provided with a so-called magnet wheel that on a circular line in the circumferencial direction comprises a plurality of marks, the passing of which is detected by a non-rotatingly arranged sensor based on magnetic flux fluctuations. From the time interval between passing of two adjacently arranged marks of the magnet wheel, the microcomputer of the control device calculates the wheel rpm (rotary frequency). For this purpose, the reciprocal value of the time period (time interval) is multiplied with the reciprocal value of the number of poles along the circular line. After multiplication of the thus determined rotary frequency with the stored circumferencial length of the vehicle wheel, the vehicle wheel circumferencial velocity is determined. Based on these data the computer, also referred to as an electronic control device, also detects the change of the wheel rpm or of the wheel circumferencial velocity over time.
From German Patent Application 44 35 160 it is known that the circumferencial force acting on the wheel causes a rotation of the belt and of the tread surface relative to the rim and the parts that are in torsion-shift connection thereto, especially the magnet wheel. In the entire torsional deformation chain from the tire tread surface, the belt, the tire sidewalls, the tire bead, the rim flange, the rim bed, the rim dish, the fastening screws, the wheel rim, to the magnetic wheel, the by far greatest torsional softness is located within the tire sidewall.
With the devices for measuring the rotary frequency of a rotating vehicle wheel known in the prior art, the rotary frequency can only be determined for the magnet wheel of the vehicle wheel. This rotary frequency coincides precisely, due to the torsionally stiff connection, with the rotary frequency of the brake drum or the brake disc of the wheel, especially during phases of acceleration changes, but does not coincide with the rotary frequency of the tire tread surface. However, this is the decisive factor in regard to slip on road surfaces. The inventors have recognized that the known ABS systems function much better when in addition to the rotary frequency of the brake drum, respectively brake disc the rotary frequency of the tire tread surface is also determined. This overcomes the prejudice in regard to the concept that only one single rotary frequency is present at the wheel. The inventors have recognized instead that substantially two rotary frequencies are present, i.e., the rotary frequency of the tire tread surface and of the belt ply array (in short, belt) underneath and the rotary frequency of the rim parts, wheel hub, sealing rings, brake disk or drum and the magnet wheel, on the other hand, i.e., parts in torsionally stiff connection relative to the wheel rim or the wheel rim itself and also the tire bead that is torsionally stiff relative to the wheel rim.
Thus, it is an object of the present invention to improve a device for measuring the rotary frequency of a rotating vehicle wheel in order to provide more useful data for slip control etc..